A “brain tablet” was a cowboy term for a cigarette. Maybe they looked like brain capsules at the time, or did cowboys think the cigarettes made them smarter? The term “brain tablet” is cited from at least the 1920s.
(Historical Dictionary of American Slang)
brain tablet n. BRAIN CAPSULE
1933 American Speech (Feb.) 31: Brain tablet. Cigarette.
1936 R. Adams Cowboy Lingo 206: The cigarette itself was referred to as a “brain tablet.”
24 September 1927, Daily Northwestern (Oshkosh, WI), pg. 2, col. 3:
DIALOGUE OF COWBOYS
GREEK TO STRANGERS
San Angelo, Tex.—(AP)—A dictionary would be about as worthless as a song in a hurricane to a New Yorker trying to find his way around the ranch country of the west.
Cowboyese, the dialect of the ranges, is as intricate and snappy as New Yorkese and changes almost as rapidly. Some of the terms used in the pioneer days have come down unchanged through the years, but other influences—mainly that of the cavalry in which most of the cowhands fought in the world war—are apparent in the dialect.
What would a native of New York’s East Side do if confronted with a conversation like this:
“The top screw mounted his cutting horse, and, followed by a group of chuck eaters, started to trail a bunch of cattle. The corral rope was on his saddle, next to the sougan, and as he placed a brain tablet in his mouth, his mount began to swallow its head and soon turned the pack.”
A “brain tablet” is a cigaret.